This application is a United States national phase application filed under 35 U.S.C. xc2xa7371 claiming priority from International Application PCT/EP99/06451, filed Sep. 2, 1999, and published under PCT Article 21(2) in English.
The present invention regards a process for pickling and surface finishing of cold-rolled products or plane or long draw pieces made of stainless steel of the austenitic, ferritic and martensitic types, duplex steels, superaustenitic and superferritic steels, special alloys of nickel or nickel-chromium.
The process, which has been devised in particular for continuous production, comprises a number of operating steps, at least one of which consists of a stage of electrolytic treatment, on processing lines in which the material to be pickled may or may not undergo a pre-treatment in a molten salt bath.
Specifically, the present invention replaces the electrolytic bath in nitric acid and is followed by a treatment of passivation and/or final pickling, according to the type of material undergoing treatment.
For pickling of stainless steels that are cold-rolled and subjected to thermal treatment of annealing, numerous electrolytic pickling processes are known, among which the following are cited to provide examples:
The patent DE-A-19624436 describes a process for electrolytic pickling using only HCl as acid agent, together with ferric chloride, in a concentration of from 30 to 120 g/l. The steel strip to be pickled is made to pass between pairs of electrodes set on both faces of the strip, the electrodes of each pair having the same polarity. Arrangements of the electrodes are described in the sequence cathode-anode-cathode-cathode-anode-cathode, the elementary unit being thus represented by the ternary sequence cathode-anode-cathode. The current density is in the region of 3 to 4 A/dm2. The treatment temperature is between 50xc2x0 C. and 95xc2x0 C.
The patent DE-C-3937438 describes a pickling process using a solution containing from 5 to 50 g/l of HF and up to 150 g/l of Fe3+. Re-oxidation of Fe2+ to Fe3+ is obtained electrolytically by causing the material being pickled to function as anode in the pickling solution against cathodic counter-electrodes or using the pickling tank itself as a cathode. The anodic current density is between 0.1 and 1/A/dm2.
EP-A-838542 describes a process in which the steel strip passes vertically between pairs of counter-electrodes. A neutral electrolyte is used consisting of sodium sulphate in a concentration of 100 to 350 g/l and with a current density on the strip of between 20 and 250 A/dm2.
WO 98/26111 describes a process for the pickling of steels and titanium alloys with the use of H2SO4- and HF-based solutions containing Fe3+ or Ti4+ as oxidizing agents that form during the process by means of electrolytic oxidation of the corresponding reduced cations.
EP-A-763.609 describes electrolytic pickling of stainless steel using cells in series comprising alternately anodes and cathodes as counter-electrodes. The electrolytic solution is H2SO4-based.
JP 95-130582 uses an H2SO4-based electrolytic solution (20 to 400 g/l) containing nitrates and/or sulphates for pickling of stainless steel.
The known processes are substantially based on one of the following technologies or combinations thereof:
a) an initial treatment for conditioning the scale in molten salts (de-scaling), a subsequent treatment of electrolytic pickling carried out in nitric acid-based solutions, and finally, a chemical treatment in solutions of nitric acid or mixtures of nitric acid and hydrofluoric acid, according to the type of material to be treated;
b) an initial electrolytic treatment in sulphates, followed by a chemical treatment in nitric acid or in nitric acid/hydrofluoric acid mixtures;
c) an initial electrolytic treatment in solution of neutral sulphates, a second electrolytic treatment in solutions of nitric acid, and a final chemical treatment in solutions of nitric acid/hydrofluoric acid mixtures.
The diagram of FIG. 1 is a schematic representation of an electrolytic unit for the treatment of continuous stainless-steel strip, suitable for carrying out the process according to the present invention. The system comprises a sequence of various electrolytic units in which counter-electrodes having a cathodic function are alternated with counter-electrodes having an anodic function and are arranged along the path of the strip. As it passes through the various electrolytic units, the steel strip will assume by induction each time a polarity opposite to that of the counter-electrodes that it meets along its path.
In FIG. 1, the level of the solution is indicated by xe2x80x9cLxe2x80x9d, the supporting rollers by R, and the immersion roller by Rxe2x80x2.
As material for the anodic counter-electrodes arranged in the bath, cast iron or lead or some other material resistant to anodic attack will be used. For the cathodic counter-electrodes, stainless steel is generally used.
The current density on the stainless-steel strip in the anodic polarization stage may vary within a wide range; just to give some indication, it may range from 2 to 40 A/dm2, and in particular from 3 to 30 A/dm2. The current density on the stainless-steel strip in the cathodic polarization stage will vary according to the ratio between the cathodic and the anodic surfaces on the strip, which is generally between 1:2 and 1:6.
The cathodic current density will consequently be greater than the anodic current density (i.e., from 2 to 6 times higher).
Electrolytic treatment (in nitric or neutral sulphates) in the technologies described above constitutes the basic stage of the pickling process which enables a material with the desired surface characteristics to be obtained.
According to the technologies described previously, in the presence of electrolytic treatments with nitric acid solutionsxe2x80x94technology a) or technology c)xe2x80x94an optimal finish of the product is obtained, but there emerge the well-known environmental problems linked to the emission toxic fumes of Nox and to the presence of high concentrations of nitrate ions in the waste liquors. As regards pickling of the materials referred to herein carried out exclusively using chemical methods (as in the treatment of ferritic, martensitic and austenitic steels following on hot-rolling), these problems have already been tackled and solved by adopting nitric acid-free processes, such as those indicated in the patents EP 505606 and EP 582121. The replacement of nitric acid in electrolytic process applications typical of processes a) and c) has, instead, not yet been solved.
According to the present invention, the electrolytic treatments described using nitric acid or other possible mineral acids are replaced by treatments using sulphuric acid-based solutions and ferric ions, and a subsequent final treatment of passivation and/or pickling is carried out or otherwise, according to the type of material being treated.
The use of solutions containing sulphuric acid and ferric ions as electrolytic bath enables the following results to be obtained:
1. Elimination of nitric acid, and hence solution of the environmental problems connected thereto;
2. Better characteristics of surface finish compared to those obtainable using treatments in electrolytic nitric acid solutions;
3. Rate of pickling equal to or higher than electrolytic treatments using nitric acid solutions.